Conventional solid electrolytic capacitors are ones wherein an oxide film is formed by anodization on the surface of a valve action metal such as aluminum (Al), tantalum (Ta), and niobium (Nb), and a solid electrolyte such as MnO2, an electroconductive polymer, and the like is used as a counter electrode. The solid electrolytic capacitor is obtained by connecting a terminal to the solid electrolytic capacitor, sealing the entirety thereof in resin, etc.
FIG. 1 is a cross-sectional view showing such a conventional solid electrolytic capacitor. The explanation below is based on this drawing.
The solid electrolytic capacitor element 100 comprises a valve action metal sintered body 102, oxide film 104, solid electrolyte layer (manganese dioxide layer) 106, graphite layer 108, and silver conductive layer 110. The valve action metal sintered body 102 is an anode, the oxide film 102 is a dielectric substance, and the solid electrolyte layer (manganese dioxide layer) 106, graphite layer 108, and silver conductive layer 110 form the counter electrode.
The role of the graphite layer 108 is believed to be as follows: (1) The graphite layer 108 intercepts oxygen, and thermal degradation due to oxidation of the solid electrolyte layer 106 is inhibited thereby; and (2) The graphite layer 108 decreases the contact resistivity between the solid electrolyte layer 106 and the silver conductive layer 110.
A reduction in ESR (“equivalent series resistivity”) is being sought after as a trend in capacitor technology. To achieve lower ESR, electroconductive polymers are being used in place of the manganese dioxide that has been the raw material in conventional solid electrolyte layers. The conductivity of these electroconductive polymers is about 10 to 100 [S/cm], which is roughly 10 to 100 times higher than that of manganese dioxide.
Known attempts to reduce the ESR in a solid electrolytic capacitor have involved elimination of the graphite layer 108, and direct connection between the solid electrolyte layer 106 and the silver conductive layer 110. Theoretically, the resistivity attributable to the graphite layer 108 is eliminated thereby.
Japanese Patent Application Laid-open No. H11-135377 and No. 2005-93741 have proposed technology whereby the graphite layer 108 is eliminated, and the solid electrolyte layer 106 and silver conductive layer 110 are directly connected.
In the solid electrolytic capacitor disclosed in Japanese Patent Application Laid-open No. H11-135377, a conductive layer is formed that contains a small amount of organic compound in metal microparticles with a particle size of 10 to 500 Å (1 to 50 nm). This application asserts that because the particle size of the metal microparticles is small and a small amount of organic compound is contained therein, the metal microparticles are incorporated into the interior solid electrolyte layer, the contact surface area between the solid electrolyte layer and the conductive layer is increased, and contact resistivity is reduced thereby.
In the solid electrolytic capacitor disclosed in Japanese Patent Application Laid-open No. 2005-93741, a silver conductive layer is formed using a silver paste comprising a mixture of particles of silver powder with a mean particle size of 0.2 to 20 μm, silver nanoparticles with a mean particle size of 1 to 100 nm, and a designated binder.
It has been determined that the use of an electroconductive powder of ultrafine granules such as those disclosed in Japanese Patent Application Laid-open No. H11-135377 and No. 2005-93741 is effective in reducing the contact resistivity between the solid electrolyte layer and conductive layer, but there is also the possibility that the properties of the capacitor itself will be diminished thereby. More specifically, if the silver particles of the silver paste used to form the silver conductive layer are too fine, the silver particles can become incorporated into the interior of the capacitor, thereby causing the capacitor to short out.
It is desirable to decrease ESR and to suppress the various adverse effects.